Catalysts play a key role in increasing the efficiency of chemical synthesis and processing by lowering the reaction temperature and pressure, increasing product yield, and reducing by-product formation. Development of environmentally benign synthesis processes that eliminate toxic feed stocks, combine process steps, and result in a net reduction of pollutants and energy use rests on, to a great extent, the ability for innovation in the design of synthesis pathways and their catalyst.
CO2 utilization is an important process from the viewpoint of green chemistry. The objective of CO2 utilization is to design an efficient chemical process for conversion of captured CO2 to useful products. CO2 can participate in many chemical reactions that lead to useful products. Among these reaction processes is the dimethyl carbonate (DMC) synthesis.
DMC is an important raw material with versatile applications as a nontoxic substitute for toxic and corrosive agents such as dimethyl sulfate, dimethyl halides, and phosgene in methylation and carbonylation processes. In addition, phasing out methyl tert-butyl ether (MTBE) has led to consideration of DMC as an environmentally friendly, oxygenated fuel additive, i.e. octane enhancer, due to its high octane number, low toxicity, and quick biodegradabillity. DMC has an oxygen content three times that of MTBE (53 wt % vs. 18 wt %) so that on a weight basis only one third as much DMC is required to achieve the same oxygen level as MTBE.
DMC is a very good blending component, which has very high oxygen content (53 wt %) for environmental gasoline. Recently, automotive emission testing with DMC indicated that DMC is a more effective oxygenate than MTBE. DMC reduced total hydrocarbon and CO emission more than MTBE at the same weight percent of oxygen in the fuel. Therefore approximately 4.5 times less volume of DMC is required as compared to MTBE at the same weight percent oxygen in the fuel. Formaldehyde emission was also lower with DMC than with MTBE. DMC also exhibits good blending with octane.
DMC is classified as slightly toxic and is a more effective oxygenate than MTBE at the same weight percent oxygen in the fuel. In addition, DMC has a low emission of CO and NOx in automobile exhaust. The solubility of DMC in water is slight, whereas the solubility of MTBE is 4.3 wt %, which leads to MTBE accumulating in ground water.
Developing an environmentally friendly process and an effective catalyst is key for creating an economical and efficient technology for converting CO2 to DMC.
The DMC amount needed by a typical major refiner company to increase the oxygen content of its gasoline by 1 wt % is approximately 10,000 bbl/day. However, the total worldwide production capacity of DMC is estimated at about 1000 bbl/day.
Currently, DMC is primarily produced by oxidative carbonylation of methanol over CuCl. The main problems with this process are the low per-pass conversion, corrosion by chloride, and the presence of chloride in the DMC product. Another route for DMC production is oxidative carbonylation using nitric oxide. The major concern with this process is the use of nitric oxide. DMC can also be synthesized by (i) the reaction of methanol with phosgene, (ii) oxidative carbonylation of methanol by CO and O2 with the use of Cu and/or Pd catalysts, and (iii) co-production of DMC and ethylene glycol through the transesterification of ethylene carbonate with methanol. These routes use poisonous, flammable, and corrosive material such as phosgene, hydrogen chloride, carbon monoxide, and nitric oxide. Also, they carry potential explosion hazards.
An oxidative carbonylation process for producing DMC from CO2 and methanol has been developed. In this reaction route, a copper chloride catalyst system is used. The reaction is basically a redox system in which copper catalyst, as cuprous chloride, is oxidized by elemental oxygen, in the presence of methanol, to cupric methoxychloride, which is then reduced with carbon monoxide to form dimethyl carbonate and to restore the cuprous chloride. Both reactions take place simultaneously. The reactions of the process can be summarized as follows:
                                          2            ⁢            CuCl                    +                      2            ⁢                          CH              3                        ⁢            OH                    +                                    1              2                        ⁢                          O              2                                      ->                              2            ⁢                          Cu              ⁡                              (                                  OCH                  3                                )                                      ⁢            Cl                    +                                    H              2                        ⁢            O                                              (        I        )                                                      2            ⁢                          Cu              ⁡                              (                                  OCH                  3                                )                                      ⁢            Cl                    +          CO                ->                                            CH              3                        ⁢                          OCOOCH              3                                +                      2            ⁢            CuCl                                              (        II        )            
The overall reaction is:
                                          2            ⁢                          CH              3                        ⁢            OH                    +          CO          +                                    1              2                        ⁢                          O              2                                      ->                                            CH              3                        ⁢                          OCOOCH              3                                +                                    H              2                        ⁢            O                                              (        III        )            The process is believed to take place in a series of liquid-filled continuous stirred tank reactors, operating at approximately 393 K (120° C.) and a pressure of 27 atmospheres (2735 KN/m2). Since the oxygen is the limiting reagent, it must be fed at a carefully controlled rate. The maximum content of oxygen must not exceed 4 mol % at any point in the system to avoid the potential for explosion.
The main problems with this process are the low per-pass conversion, corrosion by chloride, and the presence of chloride in the DMC product.
A similar oxidative carbonylation route to the previous strategy has been developed. In this technology, nitric oxide (NO) is used as a redox coupling agent for the formation of dimethyl oxalate (DMO) and dimethyl carbonate (DMC). This technology has been developed and commercialized mainly to produce DMO. The DMO catalyst system was modified later to give high selectivity to DMC. A 4500 metric ton/year plant for DMC synthesis has been built. The reaction conditions of the process are in the range of 1-20 atmospheres and 323-423 K (50 to 150° C.), where the catalysts used were equimolar amounts of palladium chloride and a second metal chloride (Fe or Cu). These catalysts were co-impregnated on an active carbon support. The reactions of the process are thought to proceed as follows:
In a first step, methanol is reacted with oxygen and NO to form methyl nitrite (MN) and water:
                                          2            ⁢                          CH              3                        ⁢            OH                    +                      2            ⁢            NO                    +                                    1              2                        ⁢                          O              2                                      ->                              2            ⁢                          CH              3                        ⁢            ONO                    +                                    H              2                        ⁢            O                                              (        IV        )            
In a second step, gaseous methyl nitrite reacts with a mole of carbon monoxide over the bimetallic catalyst to form DMC and restore the original NO:2CH3ONO+CO→CH3OCOOCH3+2NO  (V)
The overall reaction is:
                                          2            ⁢                          CH              3                        ⁢            OH                    +          CO          +                                    1              2                        ⁢                          O                              2                ⁢                                                                                                      ->                                            CH              3                        ⁢                          OCOOCH              3                                +                                    H              2                        ⁢            O                                              (        VI        )            
This latter technology has particular advantages over the first noted strategy. A major advantage of the latter system lies in a dual reactor scheme, where the feed methanol and the water by-product never pass over the metal chloride catalyst. On the other hand, in the former system, water deactivation of the catalyst limits conversion to 15-20%. By separating the water from feed gas, the per pass conversion of the methyl nitrite can approach 100% without significant catalyst deactivation. Also, the latter process takes particular advantage of the fact that the redox reagent is a gas in both of its states as NO and CH3ONO. The similar species in the former process are solids, i.e. CuCl and Cu(OCH3)Cl. The simplicity of vapor/liquid separation compared to solid/liquid separation benefits the latter process. However, extreme care must be considered when mixing the three reactants (methanol, nitric oxide and oxygen) to stay outside the explosion limits of the reaction. Methyl nitrite is also highly reactive and must be handled with care. The use of the latter route also results in additional toxicity concerns due to the use of nitric oxide.
It is also known to form dimethyl carbonate (DMC) by a transesterification reaction between ethylene carbonate and methanol, with ethylene glycol as a co-product:C2H4CO3+2CH3OH→CH3OCOOCH3+C2H4(OH)2  (VII)
It also is possible to produce DMC by the methanolysis of urea. The tin-catalyzed reaction of methanol with urea to give DMC is a well known synthesis. The reactions of the process can be illustrated as follows:(NH2)2CO+CH3OH→H2NCOOCH3+NH3  (VIII)H2NCOOCH3+CH3OH→CH3OCOOCH3+NH3  (IX)
The overall reaction can be presented as follows:(NH2)2CO+2CH3OH→CH3OCOOCH3+2NH3  (X)However, this reaction is not thermodynamically favorable as the ideal gas free energy change (ΔG) for this reaction is +3.2 kcal/mol at 373K (100° C.). The first methanolysis step (reaction VIII) to methyl carbamate is favored, but dimethyl carbonate (reaction IX) is not favored. Moreover, the chemistry is thermodynamically unfavorable and an additional driving force will be required in order to achieve reasonable conversion levels.
Two other technologies also have attractive possibilities for DMC production. These are: (i) the use of supported copper on carbon catalyst, which occurs in the gas phase and avoids the need for solid-liquid separation, but the catalyst deactivation is a major problem; and (ii) the alkylene carbonate routes which are attractive because they start with two relatively low cost materials, i.e. ethylene and carbon dioxide.
The direct synthesis of DMC starting from alcohols and carbon dioxide was studied since the 1980s. This route for DMC synthesis from inexpensive feedstocks such as CO2 and methanol (as shown below) is challenging:2CH3OH+CO2→(CH3O)2CO+H2O  (XI)
It has been reported that DMC can be produced from CO2 and methanol in the presence of various catalysts, such as dialkylin dialkoxides, tin(IV), tetra-alkoxides, titanium(IV) tetra-alkoxides, bases, a mixture of palladium(II) chloride and copper(II) acetate, and thallium(I) hydroxide and alkali metal iodides. However, these reaction systems are homogeneous, which present three major problems: (a) difficulty in catalyst recovery, (b) reaction conditions of high pressure, and (c) rapid deactivation of the catalyst by process excursions.
Recently, catalytic DMC synthesis starting from carbon dioxide and methanol has been studied over zirconia (ZrO2) catalysts. The effectiveness of this catalyst was attributed to the presence of both acidic and basic sites. It was proposed that basic sites are required to activate methanol and CO2, and that acidic sites are required to supply methyl groups from methanol in the last step of the reaction mechanism. However, the selectivity and yield of this reaction was far from satisfactory.
Accordingly, a need exists for an improved process for producing dimethyl carbonate. Specifically, it would be desirable to provide an economical process for producing dimethyl carbonate from carbon dioxide and methanol, without the numerous problems associated with currently known strategies.